Helicopters are generally equipped with main engines that are used to propel the helicopter and with one or more auxiliary engines. The auxiliary engines are small gas turbines (generally referred to by the acronym APU, standing for auxiliary power unit), the main function of which is to supply non-thrust power—electrical, mechanical, hydraulic and/or pneumatic—in the flight phases in which the main engines are not in a position to do so (on the ground, in the takeoff or landing phases, in search phases, etc).
US-A1-2014/145028 describes a helicopter comprising two main thrust engines and a main system for controlling these two engines.
This helicopter also comprises a secondary thrust engine associated with a second control system independent of the main control system. This second control system controls the secondary engine according to a constant setpoint. Because of this, the secondary engine cannot be controlled using a speed error of the free turbine of the secondary engine, which has the drawback of possibly requiring balancing with the main engines of the helicopter.
It has also been proposed by the applicant, in particular in the patent application FR 2 992 024, to use the auxiliary engines not only to supply non-thrust power but also, in certain flight phases, to supply extra thrust power, in addition to the thrust power of the main engines.
In the context of such use of an auxiliary engine, for supplying both thrust and non-thrust power, one of the difficulties that is posed now is being able to control this auxiliary engine at low and controlled droop, so as to be able to adapt the thrust power delivered by this engine according to the flight conditions of the helicopter. The problem is posed in particular of being able to control the auxiliary engine without having an impact on the functioning of the main engines.